When using an optical pickup device in a CD player or the like, a diffracting element (hologram element) is utilized for reducing the number of parts comprising the optics.
An optical pickup device of this kind is shown in FIGS. 10 and 11.
As shown in FIG. 10, a laser beam emitted from a light emitting element 11 first passes through a diffracting element 12. A 0th-order diffracted ray having passed through a diffracting element 12 is focused on a disk 15 through a collimator lens 13 and an objective lens 14.
Then, a reflected ray from this disk 15 passes the diffracting element 12 in the opposite direction through the objective lens 14 and the collimator lens 13. The diffracting element 12 is divided into diffraction regions 12a and 12b being different in diffraction angle by a parting line along a track direction, as shown in FIG. 10. Hence, a 1st-order diffracted ray diffracted in the diffraction region 12a is focused on a light receiving elements 16a and 16b. On the other hand, a 1st-order ray diffracted in the diffraction region 12b is focused on light receiving elements 16c and 16d.
Individual output signals Sa-Sd from these light receiving elements 16a-16d are respectively converted into a focusing error signal FE, a tracking error signal TE and a reproduced information signal RF by means of an arithmetic circuit shown in FIG. 11. The arithmetic operation represented by the following formula, according to a kind of knife edge method, is performed on the output signals Sa-Sd using addition circuits 17 and 18 and a subtraction circuit 19 whereby the focusing error signal FE is detectable. EQU FE=(Sb+Sc)-(Sa+Sd)
The arithmetic operation represented by the following formula, according to a push-pull method, is performed on the output signals Sa-Sd using addition circuits 20 and 21 and a subtraction circuit 22 whereby the tracking error signal TE is detectable. EQU TE=(Sc=Sd)-(Sa+Sb)
The reproduced information signal RF is detectable by adding all the output signals Sa-Sd, using the addition circuits 20 and 21 and the other addition circuit 23, according to the following formula. EQU RF=Sa+Sb+Sc+Sd
The aforementioned optical pickup device, however, has its disk 15, for instance, warped so that, if an incident optical beam does not hit the disk surface vertically, it results in coma-aberration occurring in a beam spot on the disk surface, thereby causing cross talk from the adjacent tracks. With such an optical pickup device, therefore, it is necessary to provide a tilting mechanism for tilt angle adjustment according to an inclination of the disk surface for compensation thereof so that the optical beam is always incident upon the surface of the disk 15 vertically, even if the disk 15 is warped.
In the case that a tilt angle is to be adjusted by means of the tilting mechanism, it has hitherto been a usual practice to provide the tip of the optical pickup device with a tilt sensor 24 as shown in FIGS. 12(a) and (b) whereby the tilt of the disk surface has been detected.
This tilt sensor 24 is made up of a light emitting element 25 and a pair of light receiving elements 26a and 26b. The light emitting element 25 is arranged that an irradiating ray therefrom is emitted toward the disk 15, while the light receiving elements 26a and 26b are disposed on both sides of the light emitting element 25 in a radial direction with regard to the disk 15.
In this arrangement, as shown in FIG. 12(a), when the disk 15 is warped upward as being radially outward, an irradiating ray from the light emitting element 25 is reflected by the disk 15 outward and the quantity of received ray of the outer light receiving element 26b is larger than that of the inner light receiving element 26a. Also, as shown in FIG. 12(b), when the disk 15 is warped downward as being radially outward, an irradiating ray from the light emitting element 25 is reflected by the disk 15 inward so that the quantity of received ray of the inner light receiving element 26a is larger than that of the outer light receiving element 26b. Hence, when the outputs from the light receiving elements 26a and 26b are inputted into a subtraction circuit 27, shown in FIG. 13, a tilt error signal in accordance with an inclination of the disk surface is obtainable.
However, providing the tilt sensor 24 independent of the optical pickup device as mentioned above means increasing the number of parts of the device. Also, it is necessary to do initial adjustment so that the tilt error signal released from the subtraction circuit 27 with respect to a warp-free and flat reference disk is "0". This results in an increased assembly man-hours of the product.
Thus, a conventional optical pickup device has to be provided with an independent tilt sensor 24 for adjustment of tilt angle, and this meant an increased number of parts and necessity of initial adjustment, and these are naturally accompanied by a cost increase of the product.